Generally, fructooligosaccharides are oligosaccharides in which one to three fructose molecules are bound via β-bonds at positions C1 and C2 to the fructose moiety of sucrose and are indigestible sugars known for their excellent physiological functions, such as stimulation of the growth of bifidobacteria in the intestines, improvement in metabolism of cholesterols and other lipids, low cariogenicity, and stimulation of mineral absorption. Fructooligosaccharides are known to be widely distributed in nature in plants, such as onion, asparagus, and Jerusalem artichoke. Since technology for the mass production from sucrose utilizing a transfer reaction with β-fructofuranosidases derived from microorganisms has recently been established, they have been industrially produced. Currently, intracellular β-fructofuranosidases derived from Aspergillus niger are used in the industrial production of fructooligosaccharides.
Genes encoding these β-fructofuranosidases have been disclosed in WO 97/34004. However, these β-fructofuranosidases produce fructooligosaccharides as a mixture of 1-kestose, nystose, and 1-fructosylnystose and as a result the fructooligosaccharides have been manufactured and provided as syrup or powder of the oligosaccharide mixtures. If β-fructofuranosidases which selectively and efficiently produce 1-kestose or nystose as a single component can be obtained, they would provide the following advantage. Namely, by purification of 1-kestose or nystose to a high degree, followed by crystallization, it is possible to manufacture a single-component crystal fructooligosaccharide product which has excellent characteristics in terms of properties and workability while maintaining the physiological functions of the fructooligosaccharide.
On the other hand, a method for the industrial production of crystal 1-kestose using sucrose as a raw material has been disclosed, for example, in WO 97/21718. Namely, 1-kestose is produced by reacting β-fructofuranosidase with sucrose and purified to a purity of 80% or higher by chromatographic separation, after which the resulting product is used as a crystallization material to obtain crystal 1-kestose having a purity of 95% or higher. In such a method for industrial production, characteristics of enzymes for the use required are a high conversion rate from sucrose to 1-kestose and low nystose production. Similarly, in a method for the industrial production of nystose as a single component, characteristics of the enzymes required are a high conversion rate to nystose and low 1-fructosylnystose production.